Tablet support status (or, hitting jackpot with freeze break requests)

[Carlos Garnacho <carlosg gnome org>]

Hi!,

As you probably know, there's been some work towards making drawing
tablets supported in Wayland. As with other device configurability
done previously to work on wayland too, this implies moving to
libinput-based input, also on the x11 side through
xf86-input-libinput.

But this has supposed shuffling a lot of pieces around, all tablet
configuration happening in gnome-settings-daemon should be now
performed by mutter (as with other device configurability really), and
so do other miscellaneous things like mapping buttons to keycombos or
showing and OSD with the tablet button mapping.

And it's actually in x11 support where timing got a bit tight, in the
first place mutter hasn't grown yet the support to apply our tablet
configuration on x11 (I've got code started here), but also there is
some configuration toggles still missing on xf86-input-libinput that
are needed to have 100% configuration support (stylus pressure and
"keep aspect" [1] settings most notably).

So the status of my TODO to get to the full picture is:

- Pad OSD needs moving to being handled by gnome-shell
(https://git.gnome.org/browse/gnome-shell/log/?h=wip/pad-osd). Florian
promised me a review, which I expect at some point, although he jumped
into vacation since.

- gnome-control-center needs changing to modifying the same settings
that mutter listens to, and adapt to the new configuration mockups
(https://git.gnome.org/browse/gnome-control-center/log/?h=wip/garnacho/wayland-tablet)

- mutter needs to gain support for the X11 configuration, this is WIP,
but I'm optimistic that this can be sorted out for 3.22.0 if we go
ahead with all these changes and deem this a regression, it's
basically the same than we're doing currently on gnome-settings-daemon
for the most part, albeit maybe depending on brand new (read: still
undone) xf86-input-libinput features. Peter, feel free to crush my
hopes :).

- After that, we can safely shut down the wacom plugin from
gnome-settings-daemon, everything it was doing is now taken care
somewhere else. I'm not posting a branch but you can imagine the
resulting patch.

So the summary of freeze requests are:
- Pushing wip/pad-osd in gnome-shell
- Pushing wip/garnacho/wayland-tablet on gnome-control-center
- Disabling/dropping the wacom module from gnome-settings-daemon

I know this implies a lot of late changes: dropping support for an
entire driver, new UI, losing some configurability at places
(temporarily, I hope), ... so I may understand if feels uneasy. Some
"middle ground" options I might think of are:

- Still moving OSD and configuration management to mutter, although
with mixed xf86-input-libinput/wacom driver support to ensure we can
make X11 supported again ASAP.
- Wire mutter to using gsettings from gnome-settings-daemon so
x11/wayland share the same settings, and preserve the wacom plugin in
g-s-d for X11 mostly as-is. It would still be more convenient to have
the OSD in gnome-shell nonetheless.

So, how far would you let me go? :)

Cheers,
  Carlos

[1] So eg. a 4:3 tablet sacrifices some space in order to preserve
both axes unstretched if eg. mapping to a 16:9 monitor